1. Field of the Invention:
This invention relates to a solid state oscillator for use in a microwave circuit.
2. Prior Art:
One of the most troublesome problems in a microwave or a millimeter wave apparatus using a solid state oscillation device such as a Gunn diode and an IMPATT diode is how to stabilize an oscillation frequency of a microwave solid state oscillator by a simple means. There has been a known measure that a solid state oscillation device and a dielectric resonator of a large dielectric constant, a large quality factor (hereinafter referred to as Q factor) and high stability are electromagnetically coupled with each other (high stability means here that a resonance frequency of the dielectric resonator has a small temperature coefficient).
In such a conventional configuration, the dielectric resonator is disposed at a cut-off region of a first rectangular waveguide, and a second rectangular waveguide or a coaxial cable is disposed adjacent to the cut-off region. Further, a solid state oscillation device is mounted in either one of two ways: It is mounted at a place where the oscillation device is electromagnetically coupled with the dielectric resonator inside either the second rectangular waveguide or the coaxial transmission line. Alternatively, it is mounted at a place where the oscillation device is electromagnetically coupled with the dielectric resonator inside the cut-off region of the first rectangular waveguide.
An oscillation power of a stabilized oscillation frequency is taken out by an external circuit connected to the cut-off region.
In both cases the dielectric resonator is disposed in the cut-off region of the rectangular waveguide. The reason why the dielectric resonator is disposed in the cut-off region of the rectangular waveguide is as follows. Only an evanescent electromagnetic field (electromagnetic field decaying exponentially with respect to the distance) exists inside the cut-off region. The dielectric resonator only is a resonance circuit component capable of coupling with the external circuit. The dielectric resonator has a characteristic of a band-pass filter when it is disposed at a place where the evanescent electromagnetic field is not entirely decayed. By disposing the dielectric resonator between the solid state oscillation device and the external circuit, an electromagnetic power of an oscillation frequency almost solely controlled by the dielectric resonator can be taken out from the external circuit.
One example of the conventional configurations of a microwave solid state oscillator is shown in FIG. 1(a) and FIG. 1(b). FIG. 1(a) is a front view of a microwave solid state oscillator and FIG. 1(b) is a cross sectional view taken on the plane shown by the line A--A in FIG. 1(a). A conductor 5 of a column in shape is disposed at a place inside a cut-off region 4 of a rectangular waveguide and perpendicular to the H-planes of the rectangular waveguide. A solid state oscillation device 1 is disposed at a space between the column-shaped conductor 5 and the H-plane. A disc-shaped dielectric resonator 2 of a large dielectric constant and a large Q factor is positioned by a supporter 6 in such a manner that the disc plate face thereof is parallel to the E-plane. With such a configuration the solid state oscillation device 1 and the dielectric resonator 2 are electromagnetically coupled with each other and an oscillation power of a stabilized oscillation frequency is taken out from an output port 3, to which a rectangular waveguide is attached. The dielectric resonator 2 should lead to a highly stability of the oscillation frequency of the microwave solid state oscillator even for the variation of the oscillation frequency of the solid state oscillation device per se.
In such a conventional configuration, the dielectric resonator is disposed inside the cavity defined by the cut-off region, and therefore it is necessary that the dielectric resonator 2 is disposed apart from the output port 3 and the solid state oscillation device 1 by certain distances, and further the solid state oscillation device 1 must be kept apart from the terminated short-circuited plane. For instance, in case a cut-off region is formed to have a width of 1/2 of the longer side of the rectangular opening of the rectangular waveguide, a distance of about .lambda.g/2 is necessary between the solid state oscillation device 1 and the terminated short-circuited plane, where .lambda.g is a guide wavelength of the rectangular waveguide. Such spacing makes the size of the whole oscillator as large as about .lambda.g. On the other hand, there is a limit to shorten the longer side of the rectangular opening in order to shorten the size of the apparatus, since an effective unloaded Q factor decreases due to the wall proximity effect of the dielectric resonator by the conductive wall when the longer side is shortened by less than one half of the original length and a cut-off region is formed.
On the other hand, when a coaxial transmission line configuration is employed for a mounting portion of the solid state oscillation device in order to shorten the distance between the solid state oscillation device 1 and the terminated short-circuited plane, then the size of the mounting portion becomes large in the axial direction of the coaxial transmission line due to a mounting configuration. It is also difficult to accurately install the dielectric resonator and the supporter thereof inside the narrow and deep cut-off region.